Method for cleaning elements in vacuum chamber and apparatus for processing substrates

ABSTRACT

To clean an element in a vacuum chamber by causing particles sticking to the element to scatter, the present invention uses a means for applying a voltage to the element and causing the particles to scatter by utilizing Maxwell&#39;s stress, a means for electrically charging the particles and causing the particles to scatter by utilizing the Coulomb force, a means for introducing a gas into the vacuum chamber and causing the particles sticking to the element to scatter by causing a gas shock wave to hit the element, a means for heating the element and causing the particles to scatter by utilizing the thermal stress and thermophoretic force, or a means for causing the particles to scatter by applying mechanical vibrations to the element. The thus scattered particles are removed by carrying them in a gas flow in a relatively high pressure atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of and claimspriority to U.S. application Ser. No. 10/921,947, filed on Aug. 20,2004, of which the entire content is hereby incorporated by reference,with the present application also claiming priority to predecessors ofthe '947 application as follows. U.S. application Ser. No. 10/921,947 isbased upon and claims the benefit of priority from prior JapaneseApplications JP 2003-300427, filed on Aug. 25, 2003 and JP 2004-218939,filed on Jul, 27, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for cleaning elements in avacuum chamber and, more particularly, to a technique for cleaning astage, or the like, for holding, for example, a substrate to beprocessed.

2. Description of the Related Art

In the manufacturing processes of semiconductors or flat panel displays(FPDs) such as liquid crystal displays, a major concern is to preventsubstrates, during processing, being contaminated with particlesentering from outside the manufacturing equipment or generated withinthe manufacturing equipment. In particular, if the stage installed in avacuum chamber is contaminated with particles, the particles may stickto the underside of the substrate mounted thereon, and the contaminationmay spread in subsequent steps, eventually rendering the final productsdefective.

FIG. 1 shows a schematic diagram of a conventional plasma etchingapparatus. A stage 2, for holding a substrate to be processed, isdisposed inside a vacuum chamber 1, and a high-frequency power supplyunit 3 as a bias power supply unit is connected via a capacitor 4 to thestage 2 to which is also connected, via a low-pass filter 6, to anelectrostatic power supply unit 5 for holding the substrate onto thestage 2. The vacuum chamber is grounded, and its upper surface acts asan upper electrode 7. The surface of the stage 2 is coated with alumina,polyimide, or the like, and the semiconductor substrate is attractedonto it when a DC voltage is applied from the electrostatic power supplyunit 5. A focus ring 8 is mounted on the peripheral portion of the stage2 in such a manner as to encircle the substrate placed thereon. Thefocus ring is a ring-shaped plate made of a material similar to that ofthe substrate, for example, and is provided to hold a generated plasmaon the substrate. A processing gas is introduced through gas inlet ports10 of a shower head 9 disposed above the stage. Though not shown here, apump for partially or wholly evacuating the chamber is provided. In theillustrated example, it is assumed that particles P are left sticking tothe stage 2.

When performing processing in the above vacuum chamber, thesemiconductor substrate (not shown) is placed on the stage 2, and isheld on it by electrostatic attraction by applying a voltage from theelectrostatic power supply unit 5; then, a reactive gas for processingis introduced into the chamber 1 through the gas inlet ports 10 of theshower head 9, and a plasma is generated by supplying power from thehigh-frequency power supply unit 3, to perform a predeterminedprocessing. At this time, if the particles P are left sticking to thestage 2, they stick to the underside of the substrate during processing,and the contamination spreads in subsequent steps, leading to suchproblems as a reduced production yield of the finally producedsemiconductor devices.

Possible sources of such particles include, for example, those enteringfrom outside the chamber, those due to the contact friction between thestage 2 and the semiconductor substrate, and those formed by thedeposition of products of the reactive gas. In view of this, JapaneseUnexamined Patent Publication No. 2002-100567, for example, proposes amethod of keeping the stage clean by cleaning it with a brush scrubberor a wiper blade or by spraying a clean liquid or gas onto the stage.

However, since such cleaning means usually requires opening the lid ofthe chamber and thus exposing the chamber to the atmosphere, thecleaning itself can cause contamination. Further, using a brush scrubberor a wiper blade under reduced pressure is not effective in removingparticles (for examples, particles with particle size of about 10 nm);on the contrary, this runs the risk of generating new particles due tophysical friction. On the other hand, cleaning the stage with a liquidrequires a complicated structure for cleaning, and greatly reducesthroughput. Moreover, by simply spraying a gas, it is difficult tothoroughly clean the stage, because the collision cross section betweenthe particle and the gas is very small.

SUMMARY OF THE INVENTION

In view of the above problems, it is an object of the present inventionto provide an element cleaning method that removes particles from thesurface of an element in a vacuum chamber by effectively causing theparticles to scatter, a substrate processing apparatus that is equippedwith means for implementing the cleaning method, a scattering particledetecting apparatus that monitors the cleaning, a method for evaluatingcleanness, and a method for detecting the end point of the cleaning.

To solve the above problems, in a first aspect of the invention,particles sticking to the element are caused to scatter in accordancewith a permittivity difference between the element and the particles byapplying a voltage to the element.

In a second aspect of the invention, particles sticking to the elementare electrically charged, and a voltage of the same polarity as thecharge of the charged particles is applied to the element, therebycausing the particles sticking to the element to scatter.

In a third aspect of the invention, a gas is introduced whilemaintaining the vacuum chamber at a predetermined pressure, andparticles sticking to the element are caused to scatter by causing a gasshock wave to hit the element.

In a fourth aspect of the invention, particles sticking to the elementare caused to scatter by utilizing thermal stress and a thermophoreticforce induced by controlling the temperature of the element.

In a fifth aspect of the invention, particles sticking to the elementare caused to scatter by applying mechanical vibration to the element.

In a sixth aspect of the invention, while maintaining the vacuum chamberat a pressure equal to or higher than 1.3×10³ Pa (10 Torr), particlesare caused to scatter and the particles are removed by utilizing a gasflow.

In a seventh aspect of the invention, in a preprocessing step precedingthe step of removing the particles by utilizing a gas flow whilemaintaining the vacuum chamber at a pressure equal to or higher than1.3×10³ Pa (10 Torr), the particles are caused to scatter by holding thevacuum chamber at a pressure lower than 1.3×10² Pa (1 Torr).

In an eighth aspect of the invention, when causing the particles toscatter by utilizing a gas flow while maintaining the vacuum chamber ata pressure equal to or higher than 1.3×10³ Pa (10 Torr), mechanicalvibration is applied to the particles to be scattered.

In a ninth aspect of the invention, with the element heated andmaintained at a high temperature, the step of introducing a gas whilemaintaining the vacuum chamber at a predetermined pressure and ofcausing a gas shock wave to hit the element and the step of applying ahigh voltage to the element are performed simultaneously orsuccessively.

In a 10th aspect of the invention, there is provided a substrateprocessing apparatus which applies a voltage from an electrostatic powersupply unit to the stage on which the substrate to be processed is yetto be mounted, and thereby causes particles sticking to the stage toscatter.

In an 11th aspect of the invention, there is provided a substrateprocessing apparatus which, while maintaining the vacuum chamber at apredetermined pressure, introduces a gas through a gas inlet pipe towardthe stage on which the substrate to be processed is yet to be mounted,and causes particles sticking to the stage to scatter by causing a shockwave, generated by the introduction of the gas, to hit the stage.

In a 12th aspect of the invention, there is provided a substrateprocessing apparatus which passes a head cooling gas through a gas inletpipe provided to introduce a gas to an upper surface of the stage and,in this condition, heats the stage with no substrate to be processedmounted thereon up to a predetermined temperature by using a heatingmeans and thereby causes particles sticking to the stage to scatter.

In a 13th aspect of the invention, there is provided a scatteringparticle detecting apparatus which comprises: a light source forprojecting incident light into the vacuum chamber in such a manner thatthe incident light passes through a space above the element; and a lightdetector, disposed at a predetermined angle to the incident light, fordetecting scattered light occurring due to the particles.

In 14th and 15th aspects of the invention, there are provided acleanness evaluating method for evaluating the cleanness of an elementin a vacuum chamber by detecting scattered light occurring due toparticles, and a cleaning end point detecting method for detecting theend point of the cleaning of the element.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIG. 1 is a diagram showing a prior art plasma processing apparatus towhich the present invention can be applied;

FIG. 2 is a diagram showing the results of an experiment in whichparticles were caused to scatter by utilizing Maxell's stress inaccordance with a first embodiment of the present invention;

FIG. 3 is a diagram showing the results of an experiment in whichparticles were caused to scatter by applying a voltage with arectangular waveform in accordance with the first embodiment of thepresent invention;

FIG. 4 is a picture showing an image of laser scattered light due toscattering particles in accordance with the first embodiment of thepresent invention;

FIG. 5 is an explanatory diagram showing the relationship between laserlight and scattering particles in accordance with the first embodimentof the present invention;

FIG. 6 is a diagram showing the number of scattered particles as afunction of an applied voltage in accordance with the first embodimentof the present invention;

FIG. 7 is a picture showing the scattering of particles caused by a gasshock wave, at a certain pressure, in accordance with a sixth embodimentof the present invention;

FIG. 8 is a picture showing the scattering of particles caused by a gasshock wave at another pressure in accordance with the sixth embodimentof the present invention;

FIG. 9 is a diagram showing the number of particles scattering caused bya gas shock wave applied repetitively in accordance with the sixthembodiment of the present invention;

FIG. 10 is a diagram showing the number of particles scattering causedby heating in accordance with a seventh embodiment of the presentinvention;

FIG. 11 is a schematic diagram showing a scattering particle detectingapparatus in accordance with an 11th embodiment of the presentinvention;

FIG. 12 is a diagram showing the effect of ultrasonic vibrations inaccordance with an eighth embodiment of the present invention;

FIG. 13 is a schematic diagram showing a plasma processing apparatus inaccordance with a 12th embodiment of the present invention;

FIG. 14 is a diagram showing a flow of a cleaning method in accordancewith the 12th embodiment of the present invention;

FIG. 15 is a diagram showing the relationship between the internalpressure of a chamber and the number of particles according to thecleaning method of the 12th embodiment of the present invention;

FIG. 16 is a diagram showing a flow of a cleaning method in accordancewith a 13th embodiment of the present invention;

FIG. 17 is a diagram showing the effect of preprocessing in accordancewith the 13th embodiment of the present invention;

FIG. 18 is a diagram showing the relationship between the number ofparticles remaining on a wafer and the number of repetitions of particleremoval when the particle removal with preprocessing was performed inaccordance with the 13th embodiment of the present invention;

FIG. 19 is a diagram showing one step in a cleaning method in accordancewith a 14th embodiment of the present invention; and

FIG. 20 is a diagram showing the relationship between moving speed andparticles in accordance with the 14th embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before proceeding to the description of the preferred embodiments of theinvention, the principles of the present invention will be describedfirst. The present inventor et al. analyzed attraction forces actingbetween particles and the stage, conducted studies on means forseparating and scattering particles off the stage by overcoming theattraction forces, and discovered that it would be effective to utilize(1) Maxwell's stress, (2) force generated by gas shock wave, or (3)thermal stress and a thermophoretic force, or a combination of them.That is, experimental results were obtained showing that when theseforces were applied to the stage or the particles, the particles wereeffectively separated and scattered from the stage. Here, a laser lightscattering method was used to confirm the scattering of particles.

(1) Utilizing Maxwell's Stress

The present inventor et al. obtained unique experimental results showingthat the application of a voltage to an electrostatic stage causesparticles sticking to the stage to scatter, and discovered that thisphenomenon is attributable to Maxwell's stress.

Maxwell's stress is given by

$\begin{matrix}{F = {{\rho \; E} - {\frac{1}{2}E^{2}{\nabla\; ɛ}} + {\frac{1}{2}{\nabla( {E^{2}\tau \frac{\partial ɛ}{\partial\tau}} )}}}} & \lbrack {{MATHEMATICAL}\mspace{14mu} 1} \rbrack\end{matrix}$

where ρ is the amount of charge, E is the electric field, ε is thepermittivity, and τ is the density.

The first term in the above equation expresses the Coulomb force due tocharged particles. The second term indicates that a negative forceoccurs when an electric field acts at a place where the permittivitychanges, since ∇ε is the differentiation of the permittivity withrespect to the place. The third term expresses the force, due todeformation or the like, acting on a substance whose permittivity εvaries with the density τ; rubber is an example of such a substance, butwhen we consider particles existing within semiconductor manufacturingequipment, the third term may be ignored. Accordingly, the forcesexpressed by the first and second terms can be utilized.

(2) Utilizing the Force Generated by Gas Shock Wave

As a result of an experiment conducted by spraying a gas to the stage,it was discovered that particles cannot be effectively scattered bysimply spraying the gas, but can be effectively scattered under certainconditions. That is, in the experiment, the particles could beeffectively scattered when a large amount of gas was introduced, atonce, into an atmosphere held, for example, at a pressure not higherthan 1.3×10⁻² Pa (1×10⁻⁴ Torr), and, as a result of an analysis, it hasbeen found that when a large amount of gas is introduced at once with alarge pressure difference, a shock wave occurs because of the pressuredifference and, when it hits the surface of the stage, the particles areeffectively scattered. Accordingly, the force generated by a gas shockwave can also be utilized effectively as a means for scattering andremoving the particles sticking to the stage.

(3) Utilizing Thermal Stress and Thermophoretic Force

By making the temperature of the stage sufficiently higher or lower thanits normal operating temperature by using a stage temperature controlmeans, separation of particles due to thermal stress can be induced.Further, according to an experiment, when the stage was maintained at ahigh temperature while holding a predetermined pressure, particles weresuccessfully caused to scatter from the stage by the resultingthermophoretic force. In this way, the thermal stress or thethermophoretic force can be utilized for cleaning the stage. Further, inthese experiments, in situ particle measurements were performed using alaser light scattering method. It has been found that this apparatus canalso be used for monitoring the cleanness of the stage, etc.

The preferred embodiments of the present invention will be describedbelow with reference to the accompanying drawings. The description givenherein deals with the case of a plasma etching apparatus as an example,but the present invention is not limited to this particular example, butcan be applied to any apparatus, such as a film deposition apparatus,that has a stage on which a substrate is mounted for processing.Further, the stage is not limited to a stage for mounting asemiconductor substrate thereon, but may be a stage intended for anyother kind of substrate such as a substrate for a liquid crystal displaydevice. Furthermore, the stage to be cleaned is only one example, and itwill be appreciated that the present invention can be applied forcleaning any kind of element in a vacuum chamber.

EMBODIMENT 1

In this embodiment, in case where there is a large difference betweenthe permittivity of the stage surface and the permittivity of theparticles, a predetermined electric field is formed on the surface ofthe stage in accordance with the second term of Maxwell's stressequation, and the particles are caused to scatter by the resultingrepelling force.

More specifically, before the substrate to be processed is placed on thestage, a positive or negative voltage is applied to the stage by anelectrostatic power supply unit such as shown in FIG. 1. An electricfield appears at the surface via a dielectric on the surface of thestage. The strength of the electric field at the surface of the stage isconsidered to depend on the permittivity and thickness of the dielectricon the surface of the stage but, according to an experiment, a voltageapproximately equal to the applied voltage appeared, and the strength ofthe electric field did not suffer attenuation due to the presence of thedielectric. According to Maxwell's stress equation, if an electric fieldis applied when there is a difference between the permittivity of thestage surface and the permittivity of the particles, the particlesshould experience forces that cause the particles to scatter in thedirections of the electric lines of force.

FIG. 2 is a table showing the results of the experiment. In theexperiment shown in FIG. 2, a number of materials were selected for thestage, and the amount of scattering was detected for two kinds ofparticles, one made of SiO₂ and the other of CF-based polymer. Particlescattering was particularly large in the case where the stage was madeof bare silicon (permittivity ε=11) and the particles deposited thereonwere fluorocarbon (CF) based polymer particles (permittivity ε=2), andalso in the case where the stage was made of alumina (permittivity ε=9)and the particles deposited thereon were fluorocarbon (CF) based polymerparticles. In either case, the difference in permittivity is as large as9 or 7. In the other cases where the difference in permittivity is zeroor very small, particle scattering is nearly zero or small.

FIG. 3 shows the results obtained when a rectangular waveform of +2500 Vwas applied by the electrostatic power supply unit to the bare siliconstage on which CF-based polymer particles had been deposited. The solidline shows the waveform of the electrostatic voltage, and each filledcircle indicates the number of particles. As can be seen, many (morethan 60) particles were scattered the instant that the voltage wasapplied.

As shown in FIGS. 4 and 5, the scattered particles can be detected byusing laser light scattering. FIG. 4 shows a photograph taken of thescattered particles when +2500 V was applied to the bare silicon onwhich CF-based polymer particles had been deposited. As can be seen,many particles are scattering from the surface of the stage. Thephotograph was taken by projecting laser light in the form of a flatplate-like beam at a height about 3 mm to 4 mm above the stage and bycapturing the image from one side thereof using a CCD camera.

FIG. 6 is a graph showing the number of scattered particles as afunction of the voltage applied to the stage. The horizontal axisrepresents the applied high voltage, and the vertical axis representsthe number of scattered particles. At 1000 V, no scattered particleswere observed, but at 2000 V, about 10 particles were caused to scatter,while at 2500 V, more than 60 particles were caused to scatter. Thevoltage to be applied for scattering the particles depends on thepermittivity and thickness of the dielectric on the surface of the stageand the permittivity and size of the particles; here, it was also foundthat when fluorocarbon-based particles were left sticking to theelectrostatic stage having an alumina ceramic surface as used in aplasma etching apparatus, the particles were successfully scattered andremoved by applying a voltage greater than about ±1500 V.

Further, at this time, to effectively remove the scattered particles, agas such as a nitrogen gas may be passed into the chamber and be drawnoff by a pump so that the scattered particles are drawn out the chamberby being carried in the flow of the gas. The method of drawing out thescattered particles by passing a gas can be employed in any of theembodiments hereinafter described.

In the present embodiment, an electrostatic electrode was used to applythe voltage, but a dedicated power supply unit may be provided. Further,the polarity of the applied voltage may be either positive or negative,as described above. In this way, by applying this method prior to theprocessing of the substrate when the substrate is not yet placed on thestage, particles can be prevented from sticking to the underside of thesubstrate during processing.

EMBODIMENT 2

When utilizing the difference between the permittivity of the stagesurface and the permittivity of the particles as described in the firstembodiment above, the effect can be further enhanced by coating thestage surface with a material having permittivity sufficiently largerthan that of the particles expected to stick to the surface. Generally,in an environment where the stage surface is likely to be contaminatedby the stick of silicon particles, a greater effect can be obtained ifthe stage surface is coated with a material having permittivitysufficiently larger than 11.

Examples of such materials include Bi₂O₃ (permittivity 18.2), CuO(permittivity 18.1), FeO (permittivity 14.2), KH₂PO₄ (permittivity 46),KIO₃ (permittivity 16.85), PbBr₂ (permittivity>30), PbCl₂ (permittivity33.5), PbCO₃ (permittivity 18.6), PbI₂ (permittivity 20.8), Pb(NO₃)₂(permittivity 16.8), PbO (permittivity 25.9), PbSO₄ (permittivity 14.3),SrSO₄ (permittivity 18.5), TiO₂ (permittivity 85.6 to 170), TlBr(permittivity 30.3), TlCl (permittivity 31.9), Tll (permittivity 21.8),TiNO₃ (permittivity 16.5), cyclohexanol (permittivity 16.0), andsuccinonitrile (permittivity 65.9).

EMBODIMENT 3

In the first embodiment, the forces acting on the particles are exertedthroughout the application of the voltage but, as previously shown inFIG. 3, the number of scattered particles greatly increases when thevoltage changes (in particular, the instant that the voltage isapplied). To utilize this phenomenon, a voltage of rectangular waveformmay be applied repetitively to the stage. By so doing, the particles canbe efficiently caused to scatter as the voltage is applied and stopped.Since it is the change in voltage that serves to promote the scatteringof the particles, the waveform need not be limited to the rectangularwaveform, but any other waveform, such as a pulse waveform or a sinewaveform, may be used.

The reason is believed to be that particles easier to scatter arescattered at the first application of the voltage and the particlesremaining to be scattered are given another change to scatter when theapplied voltage is temporarily removed and the voltage is applied onceagain. A similar effect can also be obtained by applying an AC voltageusing an AC power supply. The higher the AC frequency, the greater theeffect.

EMBODIMENT 4

This embodiment concerns an example in which particles are caused toscatter by utilizing the Coulomb force. When the permittivity of thestage and the permittivity of the particles are approximately the same(close to each other), the force defined by the second term of Maxwell'sstress cannot be utilized, and therefore, the Coulomb force expressed bythe first term is utilized. That is, the particles on the stage aredeliberately charged, and a voltage of the same polarity as the chargeof the charged particles is applied, thus causing the particles toscatter by the electrostatic repulsion. Here, to charge the particles onthe stage, a plasma is generated in a space above the stage on which thesubstrate is not yet to be placed. Charged particles of the generatedplasma reach the stage, thus charging the particles on the stage. Asuitable gas, such as argon, helium, oxygen, nitrogen, etc., can be usedas the gas for generating the plasma, but the gas must not contain anysubstance that can essentially corrode the material of the stage, andcontrol parameters (power, pressure, flow rate, etc.) must be selectedso that the surface of the stage will not be etched by physicalsputtering.

As the stage is negatively charged by a self-bias voltage, the particleson the stage are also negatively charged. Accordingly, by applying anegative voltage to the stage, the particles can be scattered off thesubstrate.

EMBODIMENT 5

In the fourth embodiment described above, the particles sticking to thestage are negatively charged by using a plasma, but the method ofcharging the particles is not limited to this particular example.Rather, any other suitable method may be employed, for example, a methodthat positively charges the particles by emitting photoelectrons byapplying ultraviolet light or vacuum ultraviolet light, a method thatpositively or negatively charges the particles by applying ions, or amethod that positively charges the particles by emitting photoelectronsby applying an X ray or a soft X ray. By charging the particles usingsuch a method, and applying a voltage of the same polarity as the chargeof the charged particles to the stage, the particles can be effectivelycaused to scatter.

EMBODIMENT 6

According to an experiment conducted by the present inventor et al.,when a large amount of gas was introduced in a short time into a vacuumchamber held at a pressure not higher than about 1.3×10⁻² Pa (1×10⁻⁴Torr), a shock wave with a maximum speed reaching the speed of sound wasgenerated by the pressure difference, and particles were efficientlyscattered by causing the shock wave to hit the stage. Here, during theintroduction of the gas, the gas was constantly drawn off by a vacuumpump.

For example, an N₂ gas was introduced at a pressure approximately equalto atmospheric pressure into the vacuum chamber in which bare siliconwith SiO₂ particles sticking thereto was placed. The N₂ gas wasintroduced using the shower head disposed above the stage. FIGS. 7 and 8are diagrams each showing, by way of example, the result obtained whenthe N₂ gas was introduced while increasing the pressure of the stagevacuum chamber by utilizing chamber leakage.

FIG. 7 shows the scattering of particles when the pressure of the vacuumchamber was 6.7×10⁻² Pa (5.0×10⁻⁴ Torr). FIG. 8 shows the scattering ofparticles when the pressure was 1.3×10² Pa (1.0×10⁻⁰ Torr). Each diagramshows an image of laser light scattering, captured for three secondsstarting from the introduction of the N₂ gas.

It is shown that, to scatter many particles, the pressure must be heldat about 1.3×10⁻² Pa (1×10⁻⁴ Torr) or lower, and that, at 1.3×10² Pa(1.0×10⁻⁰ Torr), hardly any effect is obtained that causes the particlesto scatter. Further, according to the experiment, it was found that thescattering of particles occurred immediately following the introductionof the gas, causing 60 to 70% of the particles to scatter.

FIG. 9 shows the results of an experiment conducted to verify theparticle scattering effect of the N₂ gas; here, after making SiO₂particles stick to the bare silicon, as in the above example, the N₂ gaswas introduced at 1.3×10⁻² Pa (1×10⁻⁴ Torr). In this example, the amountof particle scattering was evaluated by capturing the light scattered bythe particles and calculating the luminance value. The vertical axisrepresents total grayscale value, i.e., scattering intensity. Accordingto the experiment, 60 to 70% of the particles were scattered at thefirst introduction of the gas, and a small amount of particle scatteringoccurred at the second introduction; however, at the third introductionof the gas, hardly any scattering occurred. This means that the gasshould be introduced twice to accomplish the particle scattering andremoval process.

Any suitable gas such as nitrogen, oxygen, argon, etc. can be used asthe gas to be introduced here. The shape and position of the holethrough which the gas is introduced should be determined so that theshock wave can reach the particles. When introducing the gas through theshower head, the most effective result can be obtained if the showerhead is formed with a large number of closely spaced small holes so thatthe shock wave from the shower head hits the entire stage, but even whenthe existing shower head is used, a marked effect can be obtained, since60 to 70% of the particles can be scattered as described above.

EMBODIMENT 7

This embodiment utilizes the thermal stress or the thermophoretic force;that is, by making the temperature of the stage sufficiently higher orlower than its normal operating temperature by using a stage temperaturecontrol means, separation of particles due to thermal stress can beinduced. Further, by maintaining the stage at a high temperature whileholding a predetermined pressure, the particles can be moved away fromthe stage by the resulting thermophoretic force.

Here, thermophoresis refers to the phenomenon in which a body in a gashaving a temperature gradient experiences a greater momentum frommolecules on the higher temperature side than from molecules on thelower temperature side, and moves toward the lower temperature side bybeing subjected to a force acting in the direction opposite to thetemperature gradient; the thermophoretic force is dependent on theinternal pressure of the chamber and on the temperature gradient in thevicinity of particle surface.

FIG. 10 is a graph showing the results of an experiment in which theparticles were caused to scatter by heating the stage. In thisexperiment, Si with SiO₂ particles sticking thereto was used as thestage. The pressure was 1.3×10² Pa (1 Torr), and a nitrogen gas wasintroduced through the upper shower head in order to maintain the showerhead disposed above the stage at low temperature. The horizontal axisrepresent the temperature difference, and the vertical axis representsthe number of particles counted for one minute. As can be seen from thefigure, the scattering of particles began when the temperaturedifference increased to about 50° C., and a considerable number ofparticles were scattered when the difference exceeded 250° C.

According to another experiment in which the stage was heated whilevarying the pressure, hardly any scattering was observed at 1.3 Pa (0.01Torr), which shows that the scattering of particles is stronglyinfluenced by the thermophoretic force. According to still anotherexperiment conducted, the scattering particles presumably have initialvelocity, and it can be said that the particles are separated from thestage by the resultant of the thermal stress and the thermophoreticforce, and are caused to scatter by the thermophoretic force. In thisembodiment, the temperature gradient was increased by introducing thenitrogen gas into the shower head which also functions as the upperelectrode, but it will be appreciated that other suitable means may beused to increase the temperature gradient.

EMBODIMENT 8

The scattering of particles can be promoted by applying ultrasonicvibrations to the surface of the stage. That is, the sticking ofparticles to the substrate can be loosened by applying ultrasonicvibrations. Accordingly, when used in combination with any one of thefirst to seventh embodiments, the application of ultrasonic vibrationscan serve to scatter the particles more effectively. Any suitable methodcan be employed to apply ultrasonic vibrations, a typical example beinga method that connects a piezoelectric element to a portion contactingthe stage via a rigid part and applies a voltage to the piezoelectricelement.

Further, by only applying mechanical vibrations such as ultrasonicvibrations, the scattering or separation of particles occurs. FIG. 12shows an experimental example showing the particle scattering effectachieved by the application of ultrasonic vibrations. A scanningparticle detector was used to detect the scattered particles. In thefigure, the horizontal axis represents the time, and the vertical axisrepresents signals counted by the detector. As shown in the figure,residual particles carried in an evacuation line are detected when thedetection is started, but the number of particles detected graduallydecreases as the time elapses. However, when vibrations generated by aultrasonic wave are applied in the periods shown (the period from about30-second to 130-second points and the period from about 150-second to180-second points), larger numbers of particles than the particlesdetected when the detection was started are caused to separate orscatter. It is shown that during the time period that the ultrasonicvibrations are applied, scattered particles are detected intermittentlywithout any appreciable drop in the number detected. Since very fewparticles are detected during the time that the ultrasonic vibrationsare not applied, it can be seen that application of the ultrasonicvibrations is quite effective.

Further, not only by applying ultrasonic vibrations, but also byapplying mechanical vibrations caused by the movement of a componentmember, the sticking particles can be caused to scatter or separate. Inparticular, the stage is often constructed so as to be movable up anddown in the chamber, and it has been found that during the movement ofthe stage or when the moving stage comes to a stop, mechanicalvibrations occur, producing a great effect in causing the particles toscatter or separate. This will be described in detail later.

EMBODIMENT 9

Further, by combining the methods so far described, the particle removaleffect can be multiplied. All possible methods may be combined, orseveral selected methods may be combined. The methods may be combined inany suitable way; for example, the methods that can be carried outsimultaneously may be carried out simultaneously or sequentially. Themethods that cannot be carried out simultaneously should be carried outsequentially. Further, the methods of the respective embodiments may becarried out repeatedly, or a combination of the methods of someembodiments may be carried out repeatedly; in either case, a highlyeffective result can be obtained.

For example, first a gas is introduced and the force generated by ashock wave is applied to the particles (sixth embodiment), andthereafter a high voltage is applied (second and third embodiments),while continuing to heat the stage (seventh embodiment); these processesmay be carried out repeatedly. Alternatively, these processes may becarried out simultaneously and repeatedly. In particular, when utilizingthe gas shock wave, the process should be repeated twice, as previouslydescribed.

EMBODIMENT 10

The first to ninth embodiments have each been described as providing astage cleaning method, but a similar effect can also be obtained if thecleaning method is applied for cleaning other components, for example,the focus ring, included in the stage. Further, a similar effect can beobtained if the method is applied for cleaning other elements in thevacuum chamber that need cleaning.

EMBODIMENT 11

In carrying out the method of the present invention, the cleanness ofthe stage can be evaluated by detecting scattered particles using aparticle detecting apparatus such as shown in FIG. 11. Further, the endpoint of the cleaning can be detected by detecting that the number ofparticles has dropped below a predetermined number.

FIG. 11 shows the scattered particle detecting apparatus which observesscattered laser light. A stage 110 for mounting a substrate thereon isinstalled in a vacuum chamber 100. Laser light R from a laser lightsource 20 is passed through an optical system 30 such as a lens andenters the process chamber through an entrance window 120. The laserlight R passing through the optical system 30 is shaped so as to form aflat plate-like beam in a space above the stage 110. The laser light Rpropagates in the space above the stage 110, while scattered light S,reflected by the particles scattered by the method of the presentinvention, enters a CCD camera 40 through an exit window 130. The laserlight R propagated straight in the space above the stage 110 enters abeam damper 140 where the light is absorbed. The scattered light S thatentered the CCD camera 40 is converted into an electrical signal whichis supplied to an information processing apparatus 50 such as a personalcomputer, and an image of scattering particles is displayed on a displaypart 51. In the present embodiment, the image is captured as a movingimage which varies with time, but the image may be captured as a stillimage. Control information from a process equipment control panel 60 issupplied via an A/D converter 70 to the information processing apparatus50 which, based on the supplied information, controls the laser lightsource 20 and the CCD camera 40 via a pulse generator 80.

The laser light R emitted for entrance into the chamber 100 iscontrolled so that the light enters at a position aligned so as to beable to detect the scattered particles accurately. For instance, todetect the scattered particles near the stage, the laser light should bemade to enter at a height of 3 mm to 4 mm above the stage, and to detectthe particles scattering higher than that, the laser light should beshaped to be high enough to cover the higher portion.

The light source is not limited to the laser light source, but a lampmay be used as the light source. For the light detector, any suitabledevice, such as a photomultiplier, can be used. The CCD camera as thedetector is arranged so as to capture the scattering light S scatteringin a direction perpendicular to the incident light R, but may bearranged at some other angle, or alternatively, a plurality of detectorsmay be arranged at suitable angles.

FIGS. 4, 7, and 8 show examples of captured images; as can be seen, thescattered particles are clearly captured.

EMBODIMENT 12

While studying the cleaning process for separating particles from thewall surfaces of the chamber and removing the separated particles bycarrying them in a gas flow, it has been found that, to effectivelycarry the particles in the gas flow, the internal pressure of thechamber must be maintained not lower than a certain value (1.3×10³ Pa(10 Torr)). In the step of separating the particles, any means accordingto the present invention may be used, but in the case of a vacuumchamber, such as a process chamber, that has a mechanism forelectrostatically holding a wafer, the means for separating theparticles by utilizing Maxwell's stress occurring due to the applicationof a high voltage can be employed. Examples of the vacuum chamberinclude, in addition to the process chamber, vacuum transfer chamberssuch as a load lock chamber, transfer chamber, cassette chamber, etc.

FIG. 13 shows one example of an apparatus for implementing the cleaningprocess of the present embodiment. The diagram of FIG. 13 corresponds tothe diagram of the plasma etching apparatus shown in FIG. 1, except thata vent line, an evacuation system, and a wafer loading gate, omitted inFIG. 1, are added; therefore, the same parts as those in FIG. 1 aredesignated by the same reference numerals. The vent line 13 in thepresent embodiment is a passage for passing therethrough a purge gassuch as a nitrogen gas, and comprises a pipe and a valve but does nothave an orifice structure such as that of a flow rate control device.The vent line 13 can also be used as a passage for introducing areactive gas; in that case, the purge gas is introduced through theshower head 9. In this case also, no orifice structure is provided inthe passage constructed as the vent line. The reason is that, if anorifice structure is provided, the gas flow may be impeded leading to aninability to generate a shock wave. The evacuation system comprises aturbo molecular pump (TMP) 14 as the main pump, behind which is provideda dry pump (DP) 15 as a roughing vacuum pump. Further, the waferload/unload gate 17 is provided in the present embodiment.

FIG. 14 shows the cleaning process sequence according to the presentembodiment. When the process is started, first in step S1 an automaticpressure control valve (APC) (not shown) is closed, thus closing themain evacuation line of the turbo pump 14 while opening the roughingvacuum pump line 16 of the dry pump (DP) 16.

Next, in step S2, a nitrogen gas is introduced through the vent line 13at a flow rate as high as, for example, 70,000 cc per minute. Theintroduction of the large amount of nitrogen gas through the vent line13 causes a rapid increase in pressure, thus causing the particles inthe chamber 1 to separate. The separated particles are carried awaythrough the roughing vacuum pump line 16.

In step S3, the internal pressure of the chamber stabilizes at a certainvalue depending on the performance of the roughing vacuum pump 15 andthe flow rate of the nitrogen gas. In this condition, in step S4 apositive or negative high voltage is repetitively applied to the stagefrom the electrostatic power supply unit 5. For example, +3 kV and 0 Vare repetitively applied. Here, the particles sticking to the insidewalls of the chamber are separated from them in accordance withMaxwell's stress, as previously described. The separated particles arecarried away together with the nitrogen gas. After the DC high voltagehas been applied a predetermined number of times, the introduction ofthe nitrogen gas is stopped in step S5. As the roughing vacuum pump lineis left open, the roughing vacuuming continues.

In step S6, the valve in the roughing vacuum pump line is closed, andthe APC is opened to evacuate the chamber through the main vacuum lineto a predetermined pressure, for example, 1.3×10⁻² Pa (0.1 mTorr), bymeans of the turbo pump 15. The entire flow is repeated as needed.

To verify the effectiveness of this cleaning method, the number ofparticles passed through the evacuation line (roughing vacuum pump line)was detected by the laser light scattering method described in the 11thembodiment, while varying the internal pressure of the chamber. Theresults are shown in FIG. 15.

In FIG. 15, the internal pressure of the chamber is plotted along thehorizontal axis, and the number of particles counted is plotted alongthe vertical axis. As can be seen, when the internal pressure of thechamber is lower than about 1333.22 Pa (10 Torr), no particles aredetected in the evacuation line. When the pressure exceeds about 1333.2Pa, particles begin to be detected, and thereafter, the number ofparticles removed increases as the internal pressure of the chamberrises.

It has been discovered that the reason that there are no particlespassing through the evacuation line at pressures lower than about1333.22 Pa (10 Torr) is because the gas viscous force imparted to theparticles is small when the pressure is low. Accordingly, when carryingaway the particles, the effectiveness increases as the internal pressureof the chamber is raised, and it is preferable to set the chamberpressure, for example, at 6.7×10³ Pa (50 Torr) or higher.

The means employed in step S4 to separate the particles was the highvoltage application which utilizes Maxwell's stress, but instead, any ofthe previously described particle separation method may be used here.That is, use may be made of the Coulomb force or of the shock wavegenerated by a rapid introduction of a gas, or the thermal stress orthermophoretic force may be used by controlling the temperature of thestage, and in addition, mechanical vibrations may be applied.

EMBODIMENT 13

In the 12th embodiment described above, as priority is given to carryingaway the particles by utilizing the gas flow, the application of thehigh voltage for separating the particles is performed in a relativelyhigh pressure atmosphere. However, it is known that if the particles areto be separated or scattered by making effective use of Maxwell's stressoccurring due to the high voltage application, the efficiency increaseswhen the high voltage application is performed in a low pressureatmosphere. Further, as described in the sixth embodiment, thescattering of particles utilizing the gas shock wave can also beperformed more efficiently at lower pressures.

In view of this, in the present embodiment, provisions are made toperform the cleaning process of the 12th embodiment after performing theintroduction of the purge gas and the application of the high voltage ata low pressure as preprocessing steps. That is, in the preprocessingsteps, the particles are separated from the inside walls of the chamberin a low pressure atmosphere, and after that, the pressure is increasedand the separated particles are carried away. This enhances the particleseparation effect as well as the removal effect of the separatedparticles.

FIG. 16 shows a flow illustrating the preprocessing steps of the presentembodiment. When the preprocessing is started, first, in step S11, theinternal pressure of the chamber is controlled to the pressure used inthe actual process (for example, 0.2 Pa (150 mTorr) and nitrogen gas isintroduced. Here, the main evacuation line is used, and the chamber isevacuated by the turbo pump 14 and maintained at the predeterminedpressure. In this case, the separation of particles by an impact forcealso occurs more effectively.

Next, in step S12, the high voltage application, which utilizesMaxwell's stress, is performed in order to separate the particlessticking to the inside walls of the chamber. The method of the highvoltage application is the same as that employed in step S4 in FIG. 13.However, in step S4 in FIG. 13, the pressure was 6.7×10³ Pa (50 Torr),but the pressure in this preprocessing step is 2.0 Pa (0.15 Torr).

In step S13, the introduction of the nitrogen gas is stopped, and thechamber is evacuated to about 1.3×10⁻² Pa (0.1 mTorr) by the turbo pump.Then, the process is repeated again, as needed. When the preprocessingis completed after being repeated a predetermined number of times, theprocess proceeds to the flow of FIG. 14 (12th embodiment). When the mainprocess of the 12th embodiment is performed after performing thepreprocessing, a larger number of particles can be separated orscattered and thus removed than would be the case if the preprocessingwere not performed.

The high voltage application which utilizes Maxwell's stress has beendescribed as being the means used in the preprocessing step to separatethe particles, but instead, use may be made of the Coulomb force or ofthe shock wave generated by a rapid introduction of a gas, or thethermal stress or thermophoretic force may be used by controlling thetemperature of the stage and, in addition, mechanical vibrations may beapplied.

FIG. 17 is a graph showing how the number of particles varies when thepreprocessing is performed in comparison with the case where thepreprocessing is not performed. In the figure, the horizontal axisrepresents the number of times that actual etching was carried out, andthe vertical axis represents the number of particles remaining on thewafer. The number of times “1” corresponds to the initial condition ofthe chamber, showing that nearly 3,000 particles were initially present.Thereafter, up to the number of times “7”, actual etching was carriedout while performing the particle removal process without preprocessing;then, between the number of times “7” and “8”, the particle removal wasnot performed, and from the number of times “8” to “11”, the particleremoval process with preprocessing was performed.

As shown in FIG. 17, as the particle removal process withoutpreprocessing was repeated, the number of particles decreased down toabout 1000, but thereafter, the number of particles did not decreasefurther even when the process was further repeated. After that,according to a series of experiments conducted in the same chamber, theparticle removal process was not performed between the number of times“7” and “8” and, after the condition returned to the initial conditionshown at the number of times “8”, the particle removal process withpreprocessing was performed, as a result of which the number ofparticles could be reduced to 500 or less. In the example of FIG. 17, asthe experiment was started in the condition in which a large number ofparticles were present, many particles remained unremoved even after theparticle removal process with preprocessing was performed.

FIG. 18 shows correlation between the number of particles remaining onthe wafer and the number of repetitions of the particle removal processwith preprocessing when the mass production process was performed usingconventional mass production equipment by performing the particleremoval process with preprocessing according to the present invention.The horizontal axis represents the number of repetitions of the particleremoval (NPPC: Non-Plasma Particle Cleaning) process with preprocessing,and the vertical axis represents the number of particles counted.Immediately after the apparatus was started up, nearly 140 particles ofdiameter 200 nm or larger (≧200 nm φ) were present, but when the processwas performed and the particle cleaning process with preprocessing wasrepeated three times, the number of particles decreased to about 10,achieving a condition generally known as “particle spec”, i.e., lessthan 20. In this way, when there is contamination due to particles, forexample, immediately after the startup of the apparatus, thecontamination due to particles can be greatly reduced by performing theprocess of the present embodiment in place of the traditionally employeddummy run or seasoning or pump and purge.

EMBODIMENT 14

As described in the eighth embodiment, the scattering of particles canbe induced by applying mechanical vibrations. The present inventor etal. have found that the scattering of particles, caused presumably bymechanical vibrations, also occurs during the movement of the waferstage or when the stage comes to a stop. Not only the scattering ofparticles off the wafer stage, but also the separation of particles offthe inside walls, including the upper electrode disposed opposite thewafer stage, was observed. Vibrations due to the movement of the waferstage can also be transmitted via a gas remaining in the chamber. In thepresent embodiment, the separation effect is enhanced by introducing awafer stage driving sequence in the particle removal process describedin the 12th embodiment. The flow of the present embodiment is the sameas the flow of the 13th embodiment (FIG. 13), except that step S35 isadded between step S3 and step S4.

FIG. 19 shows step S35. After the pressure is maintained at about6.7×10³ Pa (50 Torr) by introducing the nitrogen gas in step S3 (FIG.13), the wafer stage is driven repeatedly in step S35 and is thus movedup and down a plurality of times, before proceeding to the high voltageapplication in step S4. The vibrations generated cause the particlessticking to the inside walls of the chamber to separate from them, ormake the particles easier to separate, facilitating their separation inthe subsequent high voltage application step.

When the particles were observed by the laser light scattering method(11th embodiment) while moving the wafer stage, scattering particleswere observed the instant that the wafer stage stopped moving upward.This is because the sticking of the particles is temporarily loosened bythe mechanical vibrations occurring the instant that the wafer stagecomes to a stop, and the particles sticking to the wafer stage scatterupward by inertia, while the particles sticking to the upper electrodedrop by gravity. The particle separation effect at this time is greaterthan that achieved by the high voltage application, and the separatedparticles are effectively carried by passing a gas such as a nitrogengas at a pressure of 1.3×10³ Pa (10 Torr) or higher.

FIG. 20 shows the relationship between the number of particles and themoving speed of the wafer stage when the wafer stage is moved upward. InFIG. 20, the horizontal axis represents the moving speed of the waferstage, and the vertical axis at left represents the particle observationratio, while the vertical axis at right represents the accelerationsensor value. The particle observation ratio is the ratio of the numberof times particles were observed to the number of times the stage wasdriven, and is proportional to the number of particles separated. Theacceleration sensor value indicates the vibration caused by the stoppingof the wafer stage. As can be seen from the figure, to achieve theeffect of the present embodiment, a higher moving speed is moredesirable. This is because the kinetic energy of the wafer stage worksas the energy that causes the particles to separate and, as the kineticenergy is proportional to the mass of a moving body and to the square ofits velocity, a greater effect can be obtained when the wafer stage oflarge mass is moved at high speed and is caused to stop. As theacceleration sensor value in FIG. 20 shows, the higher the moving speedis at the time just before the stopping, the greater is the vibration.

While the present embodiment has utilized the vibrations occurringduring the driving of the wafer stage, it will be recognized that notonly the vibrations caused by the movement of the wafer stage but, ifthere is any other moving member in the chamber, the vibrations causedby the movement of such a member can also be utilized. For example, usecan also be made of the vibrations occurring when driving a rotatingmechanism for a magnet provided to adjust the magnetic field to beapplied to the plasma, an up-down moving mechanism for a pin provided onthe wafer stage for transferring a wafer, or an open/close mechanism fora shutter provided in the wafer load/unload gate. If there is no suchdriving member that generates vibrations in the chamber, a vibrationgenerating unit, for example, a unit having such a structure as animpact driver, may be installed to generate the necessary vibrations.

The method of utilizing the mechanical vibrations of the driving memberscan be applied not only to the 14th embodiment described here, but alsoto the preprocessing in the 13th embodiment. Further, as the applicationof mechanical vibrations facilitates the scattering or separation ofparticles, the method may be used in combination with any particlescattering or separating means described in the present invention.

1. An element cleaning method which causes particles sticking to anelement in a vacuum chamber to scatter, comprising the steps of:electrically charging said particles sticking to said element; andapplying to said element a voltage of the same polarity as the charge ofsaid charged particles, and thereby causing said particles sticking tosaid element to scatter.
 2. An element cleaning method as claimed inclaim 1, wherein said charging step is a step for generating a plasma ina space above said element.
 3. An element cleaning method as claimed inclaim 1, wherein said charging step is a step for applying ultravioletlight or vacuum ultraviolet light onto a surface of said element.
 4. Anelement cleaning method as claimed in claim 1, wherein said chargingstep is a step for applying electrons, positrons, or ions to a surfaceof said element.
 5. An element cleaning method as claimed in claim 1,wherein said charging step is a step for applying an X ray or a soft Xray to a surface of said element.